Optical frequency division multiplexing (optical FDM) is known (refer to Japanese Laid-open Patent Publication No. 2011-215603), which enables a plurality of transmitting devices to convert electric signals into optical signals with different frequencies, multiplex the optical signals into an optical carrier, and transmit the optical carrier to a receiving device. This technique is used to cause cross phase modulation (XPM) between the optical carrier with a single frequency and the modulated optical signals having the frequencies and generated by the multiplex transmitting devices and transmit the plurality of data signals multiplexed into the optical carrier to the receiving device.
The receiving device receives the optical carrier and converts the received optical carrier into an electric signal. After that, the receiving device uses an electric filter to extract the data signals. Unlike the case where the data signals are extracted from the optical signals before the conversion, high-accuracy wavelength control is not executed between the multiplex transmitting devices and not executed by a wavelength separator. Thus, the receiving device may easily extract the plurality of densely multiplexed data signals. Optical FDM enables multiple types of data signals (such as video information, audio information, sensor information and wireless communication information) input to the plurality of multiplex transmitting devices to be transmitted through a single transmission path to the receiving device and enables the data signals to be extracted by the receiving device.